Dendrochronology

Dendrochronology (“tree chronology”) is essentially a simple method, and as children, we all have done dendrochronological research. When a tree is felled, it is possible to count the growth rings, and because we know that a tree makes a new ring every year, we can establish in which year the tree was planted.

American Astronomer A E Douglass^[1], who had a strong interest in studying the climate, developed the method around 1900. He theorized that tree rings could be used as proxy data to extend climate study back further than had previously been permissible. He was right, and the more trees that were added to the record, the greater the size of the data could be extrapolated and the more complete picture we could build of our past climate.

In each growing season, trees create a new ring that reflects the weather conditions of that growing season. On its own, a single record can tell us only a little about the environmental conditions of the time in a specific year of the growth of the tree, and of course the age of the tree at felling, but when we put hundreds and thousands of tree-ring records together, it can tell us a lot more.

Trees typically grow one ring per year. They start growing in the spring (the cells are light tan in color, known as early wood) and, as the growing season ends in the fall, the cell walls thicken (the dark band or late wood) and eventually stop growing in the winter causing a very distinct ring. .

The ring pattern that forms over the entire life of the tree reveals the climatic conditions in which the tree grew. Abundant moisture and a long growing season result in a wide ring. A dry year may result in a very narrow ring. At sites where trees are more sensitive to temperature (e.g. at high altitudes on a mountaintop or in the boreal forests of northern Alaska and Canada) then a wide ring indicates a warm year and a narrow ring indicates a cold year. Tree species vary greatly. We make the assumption that growth is annual with a distinct growing season.

Most tree species are reliable; oak is the most reliable tree type for tree rings – with not a single known case of a missing annual growth ring. Alder and pine are notorious for occasionally “missing a year” which is confusing enough without the fact that those species also sometimes “double up”, by having two rings in the same growing season.

Birch and willow are not used at all because of the erratic nature of their growth cycle. Since the changes to the climate since the industrial revolution, some of the more recent dendrochronology records have become erratic and in higher elevations, tree ring data has declined – we are seeing more variability than ever before.

Cross-dating is like patching together panoramic photos but instead, scientists use tree cores. They are able to assign each individual tree ring and the exact year of formation by matching patterns between cores from the same tree or between trees from different locations.

To eliminate individual variations in tree-ring growth, dendrochronologists take the smoothed average of the tree-ring widths of multiple tree samples to build up a ring history, a process termed replication.

A tree-ring history whose beginning- and end dates are not known is called a floating chronology. It can be anchored by cross-matching a section against another chronology (tree-ring history) whose dates are known.

A fully anchored and cross-matched chronology for oak and pine in central Europe extends back 12,460 years, and an oak chronology goes back 7,429 years in Ireland and 6,939 years in England. A comparison of radiocarbon and dendrochronological ages supports the consistency of these two independent dendrochronological sequences. Another fully anchored chronology that extends back 8,500 years exists for the bristlecone pine in the Southwest US (White Mountains of California).